HC55130IMZ datasheet(23/36 Pages) RENESAS

23 / 36 page

HC55120, HC55121, HC55130, HC55140, HC55142, HC55143, HC55150

FN4659 Rev 13.00

Page 23 of 36

June 1, 2006

capacitor will activate an internal latch prohibiting the ringing

of the line.

Figure 19 shows the sequence of events from ringing the

phone to ring trip. The ring relay turns on when both the

ringing code and ring sync pulse are present (A). SHD is

high at this point. When the subscriber goes off hook the

SHD pin goes low and stays low until the ringing control

code is removed (B). This prevents the SHD output from

pulsing after ring trip occurs. At the next zero current

crossing of the ring signal, ring trip occurs and the ring relay

releases the line to allow loop current to flow in the loop (C).

Operation of Line Voltage Measurement

A few of the SLICs in the UniSLIC14 family feature Line

Voltage Measurement (LVM) capability. This feature

provides a pulse on the GKD_LVM output pin that is

proportional to the loop voltage. Knowing the loop voltage

and thus the loop length, other basic cable characteristics

such as attenuation and capacitance can be inferred.

Decisions can be made about gain switching in the CODEC

to overcome line losses and verification of the 2-wire circuit

integrity.

The LVM function can only be activated in the off hook

condition in either the forward or reverse operating states. The

LVM uses the ring signal supplied to the SLIC as a timebase

generator. The loop resistance is determined by monitoring

the pulse width of the output signal on the GKD_LVM pin. The

output signal on the GKD_LVM pin is a square wave for which

the average duration of the low state is proportional to the

average voltage between the tip and ring terminals. The loop

resistance is determined by the tip to ring voltage and the

constant loop current. Reference Figure 20.

Although the logic state changes to the Test Active State

when performing this test, the SLIC is still powered up in the

active state (forward or reverse) and the subscriber is

unaware the measurement is being taken.

Polarity Reversal

Most of the SLICs in the UniSLIC14 family feature full

polarity reversal. Full polarity reversal means that the SLIC

can: transmit, determine the status of the line (on hook and

off hook) and provide “silent” polarity reversal. The value of

RSYNC_REV resistor is limited between 34.8k (10ms) and

73.2k (21ms). Reference Equation 39 to program the polarity

reversal time.

Transhybrid Balance

If a low cost CODEC is chosen that does not have a transmit

op-amp, the UniSLIC14 family of SLICs can solve this

problem without the need for an additional op-amp. The

solution is to use the Programmable Transmit Gain pin (PTG)

as an input for the receive signal (VRX). In theory, when the

PTG pin is connected to a divider network (R1 and R2

Figure 21) and the value of R1 and R2 is much less than the

internal 500k

 resistors, two things happen. First the transmit

gain from VRX to VTX is reduced by half. This is the result of

shorting out the bottom 500k

 resistor with the much smaller

external resistor. And second, the input signal from VRX is

also decreased by the voltage divider R1 and R2. Transhybrid

balance occurs when these two, equal but opposite in phase,

signals are cancelled at the input to the output buffer. The

calculation of the value of R2, once R1 is selected, is effected

by the line feed resistors. EQ. 40 can be used to calculate the

value of R2. Where : ZL= Line Impedance, ZTR = input

impedance of SLIC including the protection resistor, and

RP = protection resitors (typical 30).

FIGURE 19. RINGING SEQUENCE

OFF

RELAY DRIVER

RINGING CURRENT

IN LINE

SHD OUTPUT

RINGING CODE

APPLIED

RING SYNC

PULSE

RINGING VOLTAGE

(A)

(B)

(C)

FIGURE 20. OPERATION OF THE LINE VOLTAGE

MEASUREMENT CIRCUIT

TIP

RING

GEN

GKD_LVM

RING

GEN

FREQ

PULSE WIDTH

PROPORTIONAL TO

LOOP LENGTH

UniSLIC14

II500K

1.02

--------------------------

ZL + ZTR

ZL + 2ZRP

------------------------------





 R1

II500K

1.02

--------------------------

–

(EQ. 40)

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